As international horse transport becomes more widespread, some owners might wonder if the same equine influenza vaccines designed to protect horses from common domestic strains of the disease will also shield them from foreign strains they might encounter in other countries or even at large domestic horse shows. Researchers in Japan have determined through a recent study that most strains of equine influenza are "antigenically similar" to each other, meaning current vaccines should be able to protect against foreign strains as effectively as they do against domestic strains.

According to Thomas Chambers, PhD, head of the Office International des Epizooties (OIE) International Reference Laboratory for Equine Influenza--located at the University of Kentucky's Gluck Equine Research Center--has been studying and identifying different "clades," or types, of the equine influenza virus for the past 15 years. Each of these clades used to circulate within specific geographic regions; however the various clades are no longer bound to these regions, most likely due to the international movement of horses.

Virus mutation further complicates the issue of protecting horses against various flu strains: "Compared to the viruses that cause flu in other species, equine influenza viruses are notorious for mutating (a process called antigenic drift) in an attempt to evade horses' immune systems," explained Hiroshi Kida, DVM, PhD, the director of the Research Center for Zoonosis Control at Japan's Hokkaido University. "It is in this way that new outbreaks of equine influenza (can) occur, despite immunization."

To determine in current influenza vaccines are effective against the many difference clades of the virus, Kida et al. evaluated different equine influenza strains to determine if a single vaccine would be effective in protecting horses against both domestic and foreign strains of the virus.

Kida and colleagues isolated five different influenza strains from horses native to Japan and Mongolia affected by domestic flu strains and horses imported from Canada and Belgium affected by foreign flu strains (in this case from North America and Europe). They subsequently analyzed the type of immune response the viruses generated in infected horses, along with the viruses' genetic composition.

According to Kida, "The isolated viruses did belong to two different clades, but all five of the viruses were antigenically similar to each other (i.e., they prompt the same type of immune response from the body), indicating that the current equine influenza vaccines should protect against all of these types of viruses."

Thus, if horses based in the United States are vaccinated against the typical U.S. clades of equine influenza, the study indicates that they should be protected against other clades they might be exposed to at competitions or while traveling internationally.

Nonetheless, Kida warns that continued surveillance and analysis of the equine influenza viruses is needed to be certain this does not change.

The study, "Antigenic and genetic analysis of H3N8 influenza viruses isolated from horses in Japan and Mongolia, and imported from Canada and Belgium during 2007-2010," is scheduled to be published in an upcoming edition of the journal Archives of Virology. The abstract is available online.

About the Author

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she's worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from TheHorse.com. Learn More